1. Field of the Invention
The present invention relates to an apparatus for obtaining an information relating to a propagation velocity of a pulse wave which propagates through an artery of a living subject, for example, a pulse-wave propagation time or a pulse-wave propagation velocity.
2. Related Art Statement
There have been proposed various pulse-wave propagation information obtaining apparatuses for obtaining an information relating to a propagation velocity of a pulse wave which propagates through an artery of a living subject. The pulse-wave propagation information is used for estimating a blood pressure, a degree of an arterial sclerosis, or a peripheral resistance of the subject, or the like.
The pulse-wave propagation information includes a pulse-wave propagation velocity and/or a pulse-wave propagation time. Since the pulse-wave propagation velocity is determined by dividing a propagation path (distance) through which the pulse wave propagates by the pulse-wave propagation time, it is needed to determine the pulse-wave propagation time. The pulse-wave propagation time is determined as follows: First, a first and a second signal detecting device are worn on a first and a second portion of the subject, respectively, and detect a first signal produced in synchronism with a heart beat of the subject and a second signal produced in synchronism with the heart beat of the subject, respectively. Then, a time difference between a time when the first signal is detected and a time when the second signal is detected is determined. The time difference is defined as the pulse-wave propagation time.
For example, a phonocardiographic transducer worn on a chest of the subject is employed as the first signal detecting device, and a second heart sound II detected by the phonocardiographic transducer is employed as the first signal. A carotid pulse sensor worn on a cervix of the subject is employed as the second signal detecting device, and a notch of a carotid pulse wave detected in synchronism with the detected second heart II by the carotid pulse sensor is employed as the second signal. The notch of the carotid pulse wave is defined as a point at which the amplitude of the carotid pulse wave (i.e., the intensity of the second signal) starts increasing after decreasing from the maximum amplitude of the carotid pulse wave. A time difference between a time when the second heart sound II is detected and a time when the notch of the carotid pulse wave is detected is determined as the pulse-wave propagation time. It is generally understood that the pulse-wave propagation time determined based on the second heart sound II and the notch of the carotid pulse wave is suitable for evaluating a pulse-wave propagation time of a central portion of the subject. The carotid pulse sensor for detecting the carotid pulse wave is disclosed in Laid-Open Publication No. 10-146322 of unexamined Japanese Patent Application.
However, it needs a great skill to wear the carotid pulse sensor on an appropriate portion of the cervix of the subject. If the carotid pulse sensor is not worn on the appropriate portion of the cervix of the subject, an accurate carotid pulse wave may not be detected, so that an incorrect notch of the carotid pulse wave may be detected. Therefore, an accurate pulse-wave propagation time may not be determined.
Moreover, in the case where a pulse-wave detecting device worn on a different portion of the subject than the cervix is employed in place of the carotid pulse sensor, a notch of the detected pulse wave may be employed as the second signal. However, if the notch of the pulse wave is obscure, a time when the notch is detected can not be determined accurately. Therefore, an accurate pulse-wave propagation time can not be determined.